For many years, the sealed metal can has been the primary means of preserving and storing food unrefrigerated and ready for consumption. Metal cans, while serving their primary purposes of preservation and storage, have certain limitations. For example, if the full contents of the metal can are not used at the time of opening, the remainder is usually placed in a separate plastic dish for storage in the refrigerator rather than placing the partially-full metal can itself in the refrigerator.
A variety of plastic containers are now available to package various products including food. Plastic food containers are generally preferred over metal containers by consumers because they have a clean, hygienic image. Plastic containers for food products being distributed by the food manufacturer or packer generally must be designed to provide a long shelf life; that is, they should contain the product during distribution and on the consumer's shelf for a considerable length of time, often a year or more, without allowing spoilage of the product. Until recently, plastic containers could not perform like metal cans to preserve and store unrefrigerated foods.
For a food container to be shelf stable at ambient temperature, at least two conditions must be met. First, the inside of the container and the food itself must be kept sterile. Second, oxygen seepage or ingress must be kept to a minimum. There are three conventional methods of sterilizing food products: aseptic filling, hot filling and retorting. Aseptic filling requires that the food and container be separately sterilized, then brought together and the container sealed while maintaining sterility. This process is commonly used for low-acid liquid food products. Acid foodstuffs are usually hot filled into containers from about 190.degree. F. to about 205.degree. F. For low-acid foodstuffs, the retorting sterilization process is applied after the unsterile food and container have been packaged and sealed together.
There are many ways to seal plastic food containers. One method is by the use of a metal closure secured to the container body by a double hook seal called a double seam. This provides a hermetic seal which will withstand thermal processing retorting sterilization conditions, hot filling conditions, and the variations in pressure differential experienced by the containers during these processes. Another method of sealing is by the use of a crimp seam wherein a closure hook is crimped onto the top peripheral edge of the container body. These seams usually do not withstand retorting or hot filling conditions but are acceptable for many food products. Another method is by the use of adhesives by which a closure, rigid or flexible, is adhered to a top marginal edge portion of the container. Still another method is sealing by heat, ultrasonics or radiation wherein two sections of plastic are melted together at a reasonably low temperature to form a fusion bond. The double seam is generally the preferred means of sealing containers because it provides a highly reliable hermetic seal.
While tremendous advances have been made in creating shelf-stable containers for food products, modern technology and consumer demand have added new requirements for the container. Today's food container must be attractive in appearance, easy to use, not messy, pleasant to eat out of and, increasingly importantly, microwavable. Conventional metal cans are not recommended for use in microwave ovens, and most plastic containers must be frozen or kept cool to preserve the quality of the food product because they cannot be sterilized, do not limit oxygen ingress sufficiently or do not provide a hermetric seal of sufficient long term integrity.
One type of container which is microwavable and provides long shelf life at room temperature has a plastic container body to which a metal closure is sealed by a seam, commonly a double seam. Usually a full or nearly full open panel in the metal closure, removable or openable by an opening feature such as a line of weakness, can be removed by the consumer to expose the contents. The metal remaining on the container body at or near the seam, usually a rim thereabout, does not unduly interfere with the microwave heating of the contents.
While this basic container meets the fundamental requirements for microwavable containers, it has certain limitations. In particular, the metal remaining on the container at or near the seam is aesthetically objectionable as it tends to detract from the clean plastic appearance of the remainder of the container. Consumers desire an all plastic image. This remaining metal also may be hotter than the rest of the container after microwave heating since it may be heated both by thermal conduction from the food and by induced electrical resistance heating. However, the metal closure is still desirable for use with plastic containers because the metal seam, especially the double seam, is proven technology which has a long public health history of reliability in protecting food and keeping it sterile. It can also be applied at high speeds in equipment which is compatible with existing cannery equipment.
To protect the consumer from the hot metal or double seam, and to prevent splashing of the container contents during microwave heating, a plastic overcap is usually provided which is slipped over the top of the container by the consumer after the panel in the metal closure has been removed. This overcap must be removed after heating which exposes the potentially hot metal seam. A further potential difficulty for the consumer lies in the small inwardly projecting rim or shelf of metal that remains when the inner panel in the closure is removed. To minimize the possibility that the consumer can cut or scrape a finger on the remaining metal edge, a shelf may be provided in the container body just beneath the metal edge. However, some container designs, such as cylinders, cannot easily be provided with an inner shelf using current manufacturing processes, and the shelf by itself may not provide sufficient protection.